1. Field of the Invention
The present invention relates to a digital information restoring device and, more particularly, to a device for restoring digital information indicated by variations in the period of a reproduced signal.
2. Description of the Related Art
A known example of this kind of digital information is a time code which is recorded on a tape-shaped recording medium (hereinafter referred to as "tape") along the length thereof, and the following description will be made in relation to such a time code for the purpose of illustration.
A time code processing device 30, which has the construction shown in FIG. 1, has been proposed as a conventional type of device for reproducing the time code recorded on the tape by a tape recorder (not shown) and demodulating it into data represented by a sequence of "0" and "1".
The time code processing device 30 shown in FIG. 1 includes a demodulating circuit 31 and a PLL circuit (phase-locked loop) circuit 32. The time code processing device 30 receives as its input a time code TC represented by a two-valued waveform into which an output reproduced from the tape by the tape recorder is converted. The PLL circuit 32 generates a clock which is synchronized with the frequency of the time code TC. If it is assumed that the time code TC conforms to a BPM (bi-phase mark) method as shown in FIG. 2, the PLL circuit 32 generates a clock synchronized with the frequency of the time code TC, that is, a period T of the time code TC.
The BPM method is a digital modulation method in which in the case of data of "1", inversion occurs in the middle of the bit, while in the case of data of "0", inversion occurs not in the middle of the bit but at the boundary between bits.
The demodulating circuit 31 samples the time code TC based on the BPM method, on the basis of a clock of period (1/2)T generated by the PLL circuit 32, and compares the sample values in each period T, thereby determining whether the data in each period T is "0" or "1".
With the time code processing device 30 having the above-described arrangement, an output signal demodulated from the time code TC of BPM method and represented by a sequence of "0" and "1" can be obtained as the output signal of the demodulating circuit 31.
However, the time code processing device 30 has a number of problems. For example, if variations occur in the speed at which the tape is being reproduced by the tape recorder, the period of the time code TC also varies in proportion to the variations in the speed. For this reason, it is necessary to use as the PLL circuit 32 a circuit which can follow a wide range of frequency variations, with the result that the complexity of the circuit construction and hence the price of the entire device will increase.